The principles of electrochemical sensors are well-known and are often applied to blood gas analysis and the like. Typically, these sensors incorporate a pair of electrodes extending into an electrolyte (also referred herin to as an "acceptor"), which is in turn separated from samples by a gas permeable membrane. Gases from the sample migrate across the membrane, are taken up selectively by the electrolyte--acceptor, and are electrochemically reduced at the electrodes. Based on calibration standards, the measured current corresponds linearly to the associated partial pressure of the gas in question. For example, if the electrodes respectively constitute a platinum cathode and a silver/silver chloride anode electrode having a constant voltage imposed thereon, the electrolyte acceptor solution is a bicarbonate buffer, and the membrane is one of several known membrane materials such as PTFE film, a pO.sub.2 electrode results. Once such a cell equilibrates (i.e. partial pressure of oxygen concentration in the electrolyte acceptor is balanced with that in the sample), the electrical current across the electrodes is proportional to the oxygen partial pressure in the sample.
The principal drawbacks of electrochemical cells most frequently relate to their slow response time, their tendency to drift over passage of time, and the consequent need frequently to establish calibration standards. Objects of the present invention relate to reduction of the effect of these drawbacks.
An exciting new method of analysis has become popularly known as flow injection analysis (i.e. "FIA"). In accordance with the precepts of flow injection analysis, a sample slug is injected into a carrier stream, and passed through a flow cell which is penetrated by sensors of known response characteristics and desired selectivity. As the sample slugs pass each sensor, a selective reaction commences, and once the sample has passed by, the condition of the electrode is extrapolated or otherwise compared to known standards, to yield an indication of gas partial pressure, ion concentration, or the like element constituency in the sample. See, for example, U.S. Pat. Nos. 4,224,033 to Hansen et al.; 4,177,677 to Ruzicka et al.; 4,227,973 to Ruzicka et al.; 4,315,754 to Ruzicka et al.; 4,314,824 to Hansen et al.; 4,022,575 to Hansen et al.; and 4,013,413 to Stewart et al.
The flow injection analysis methodology provides the capacity for very rapid sequential analysis of successive samples, with the rate of successive sample analyses being limited in essence only by the inherent characteristics of the sensors used. For example, for many measurements, chemically sensitive field effect devices (i.e. those popularly known as "chemfet", "isfet", and "immunofet") show promise for instantaneous electrochemical response at least at the rates of presentation of FIA samples. Chemfet sensors will most likely not be suitable for all measurements, and indeed conventional pH, pO.sub.2, pCO.sub.2, and the like measurements may in the end be best made through conventional electrochemical sensors, or hybrid chemfet/conventional sensors. These sensors, however, tend to be rate limited by their need for equilibration, since their inherent operation involves at least partial saturation of the electrolyte-acceptor by the gases in question.
It is, therefore, a primary object of the present invention to provide electrochemical sensor designs and constructions which permit sequential sample analysis at rates such as those provided in flow injection analysis systems. It is an associated object to provide such sensors which are durable, inexpensive, and accurate for even very small sample volumes.